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So the other volume TBA is also a new line?


What makes you think that? Maybe I missed something, but the only remotely relevant quote I find is:
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both of these releases will be newly-achieved milestones for the Masterworks program when they come out!


A milestone doesn't necessarily mean a volume 1. For example, I daresay most of us would consider FF10 and X-Men 6 "milestones" in the program.

I'm tempted to guess SHIELD 1, because it would complete getting all the Silver Age lines open, and would be a milestone because of that, but I'm very very dubious of getting two Marvel Age volumes in one month. And we just got a Golden Age in December, so I'm guessing this is gonna be an Atlas volume. TTA 2 and ToS 2 wouldn't be major milestones IMO, so I think I'm going to guess Amazing Adult Fantasy 1, which will be a milestone because it's the first double-size Masterworks.